A large square-bladed shovel is ideal, but a machete is easier to carry.

Although my first shot with any gun killed a large garter snake instantly (Daisy #25 BB rifle, prone position, right into his eye, beginner's luck) I've also stood around with friends emptying our .22s into another snake who refused to die.
I gave up on shooting at snakes many years ago, if they actually need killing there are far more effective tools.